The invention relates to an automatic soldering apparatus and method thereof wherein more particularly a flux is heated and is brought in contact with a face of a board to be soldered, so as to simultaneously coat and preliminarily heat the board with the flux to thereby accelerate the soldering operation, and to dispense with the conventional pre-heater or to reduce the size thereof to thereby considerably save the electric power and accordingly reduce the size of the soldering apparatus.
So far, the process for soldering the printed circuit base board has been carried out in the order of coating the flux on the board, preliminary heating of the board, soldering the board and then cooling the board. Further the flux thus used has been a resin added with an activator such as hydrochloride of low amine such as ethyl amine and dissolved with a solvent such as isopropyl alcohol (IPA).
It is well known that the flux has the effects by the activator thereof to eliminate oxide on the metalic face of printed circuit base board and on the metalic face of electronic elements to be soldered to the board and by the resin thereof to lower the surface tension of solder and to prevent the metal from being oxidized. Further the isopropyl alcohol has an effect not only to be the solvent of the activator and resin but also to be the diluent for giving an even coating of solder to the printed circuit base board and to control the coating amount of the flux thereto. The flux is normally used in a foaming condition and only in the normal temperature.
The preliminary heating is made to eliminate the solvent in the liquid flux and to activate the activator thereof and also to preliminarily heat the printed circuit base board. More precisely the preliminary heating is firstly designed to completely volatilize the solvent from the flux so as to prevent the solvent from being otherwise explosively volatilized when the solder of high temperature contracts the solvent in the next soldering step because such a phenomenon is an obstacle against the even coating of solder, secondly to further activate the activator in the flux by the preliminary heating, and further to increase the temperature of the printed circuit base board itself to reduce the difference between the board and the heated solder to thereby mitigate the heat shock in the next soldering step. Thus the preliminary heating is generally set to heat the printed circuit base board up to 40.degree. C-120.degree. C.
Namely in reference to FIGS. 11 and 12, the conventional automatic soldering apparatus 1 is substantially composed of a device 2 for transporting the base board to be soldered, a flux coating device 3, a tank 4 for storing melted solder, a preliminary heating device 5, an air curtain producing device 6 and a cooling device 8. The preliminary heating device 5 has a heater 9 such as a nichrome wire heater or an infrared radiation heater for heating the air in a limited space to indirectly heat the board to be soldered. The solder storing tank 4 is provided to contact the board to the melted solder by way of dipping the board into the melted solder or of jetting the melted solder to the board. The cooling is done generally by a fan which blows cool air on the soldered board. This is to rapidly cool down the heated solder and simultaneously to reduce the damage to the board by the heat.
However according to the conventional automatic soldering apparatus in which the flux coating device 3 is provided to coat the flux of normal temperature on the board to be soldered, which is to be heated by the separate preliminary heating device 5, there have been the following defects:
1. Since the apparatus includes a process to deal with the flux foamed up and containing a highly ignitable organic solvent and immediately thereafter a process to heat the flux to eliminate the solvent from the flux, it is unavoidable to set the ignitable element and the heating element in the neighborhood to each other. Thus there has been a danger of fire.
2. Since the apparatus requires a considerable amount of heat not only to evaporate the organic solvent but also to heat the board to be soldered by way of a heater heating the air stratum in a limited space, the heat conduction rate is very small and also the heat capacity is small due to a low density of the air, and accordingly the heat efficiency is very low. Therefore it becomes necessary to increase the number of heaters or to extend the heating time. As the result, a great deal of electric power is required. As is seen in FIGS. 11 and 12, the preliminary heating device 5 is so bulky as to occupy about 20 percent of the length of the automatic soldering device 1.
3. According to the conventional soldering apparatus, it is practically impossible to evenly heat in a short time the board to be soldered which is of large heat capacity or of low heat conductance such as a big sized multi-layer base board or a ceramic base board. Thus there has been a possibility of producing incompletely soldered products.